1. Field of the Invention
The present invention relates to wireless communication. More specifically, the present invention relates to methods for efficiently assigning channels to femtocells, taking into account hand-off, interference, coverage area, and power control considerations.
2. Discussion of the Related Art
Because mobile telephones may be used practically everywhere, they are replacing fixed wired telephones. The article, “UMA and Femtocells: Making FMC Happen” (“Choudhury”), by Partho Choudhury and Deepak Dahuja, “White Paper, December 2007. (available: at http://www.parthochoudhury.com/UMAFemto.doc), discloses (a) that approximately 30-35% of all voice calls made over a mobile network are made by mobile subscribers at their homes, and (b) about 35% of video streaming and broadcasting service uses over cellular wireless networks in 2006 took place while the mobile subscribers are at their homes.
The trend, therefore, is for the mobile telephone to become the primary or only telephone for an individual subscriber. Furthermore, the article, “Femto Cells: Personal Base Stations” (“Airvana”), published by Airvana Inc., White Paper, 2007 (available http://www.airvana.com/files/Femto_Overview_Whitepaper_FINAL—12-July-07.pdf), reveals that those 24 years of age or younger make up to 80% of their long distance calls on wireless networks rather than over wired networks. However, there is still much to be improved in reliability, voice quality, and cost of today's mobile telephone networks in indoor environments. Typically, the mobile telephone service is more costly than a wired telephone service, and there are dead spots and poor coverage. These deficiencies result in poor customer experience, thus preventing the mobile telephone to successfully replace the wired telephone as the primary or only telephone for most subscribers.
Choudhury, Airvana, and the article “The Case for Home Base Stations” (“PicoChip”), published by PicoChip Designs Ltd., White Paper, April 2007 (available http://www.picochip.com/downloads/27c85c984cd0d348edcffe7413f6ff79/femtocell_wp.pdf) all disclose a new class of base stations (BSs) designed for indoor and personal uses, The cells served by these personal BSs have come to be known as “femtocells.” A femtocell (e.g., the home e-node B (HeNB) defined in the 3GPP standard) enables indoor wireless connectivity through existing broadband Internet connections. As described in Choudhury, femtocells are also featured in fixed-mobile convergence (FMC), where the subscribers are provided the ability to switch an active data/voice call session between home wireless network (e.g., femtocell) and a mobile network (e.g., a cellular network). As reported by Choudhury, Airvana and PicoChip, the benefits of femtocells include improved indoor coverage, reduced capital and operational expenditure, reduced bandwidth load, reduced power requirements, additional high-end revenue streams, improved customer royalty, increased average revenue per user, compatibility with existing handsets (without requiring dual-mode terminals), deployment in an operator-owned spectrum, and enhanced emergency services (since the femtocells are location-aware).
Despite these benefits, femtocell technology is still at its infancy. As identified in Airvana, the technical issues to be solved include those related to interference management (both between different femtocells and between the femtocell and the macrocell), efficient hand-off mechanisms, security, scalability, and access control. For example, co-channel implementations of femtocells—where the macrocell network and the femtocell network share the same frequency band—introduce serious challenges. Co-channel deployment of femtocells has desirable hand-off characteristics, as a mobile station (MS) may more efficiently scan the cells using the same frequency band compared to identifying the cells using other frequency bands, which require band-switching to accomplish the scanning. However, for distances that are close to the macrocell base station (mBS), severe interference from the mBS may prevent co-channel deployment.
The article, “Effects of User-Deployed, Co-Channel Femtocells on the Call Drop Probability in a Residential Scenario” (“Lester”), by Lester T. W. Ho and Holger Claussen, published in Proc. of IEEE Int. Symp. on Personal, Indoor and Mobile Radio Communications (PIMRC), pp. 1-5, September 2007, shows that the received signals from the femtocell and the macrocell in such an implementation have identical power levels at the border of the macrocell. Thus, without adequate power control, the femtocell coverage area decreases for those femtocells that are closer to the macrocell BS (mBS). However, when the femtocell coverage area falls below a certain size, the femtocell does not completely cover a user's premise, which is the preferred coverage area. A different solution is desired, under such circumstances.
The article, “Uplink Capacity and Interference Avoidance for Two-Tier Cellular Networks” (“Chandrasekhar”), by Vikram Chandrasekhar and Jeffrey G. Andrews, published in Proc. IEEE Global Telecommunications Conference (GLOBECOM), pp. 3322-3326, November 2007, derived and analyzed the uplink (UL) capacity of a co-channel femtocell network coexisting with a macrocell network (i.e., a shared-spectrum network). In a split spectrum network, the femtocell users and the macrocell users use orthogonal sub-channels. While the split spectrum network avoids interference between the macrocell and the different femtocells, the total number of users that can be supported is less than a shared spectrum network. In a shared spectrum network, a femtocell may use a sub-channel that is already used in the macrocell, so long as there is little interference between the femtocell and the portion of the macrocell network where the common sub-channel is used. In a co-channel femtocell deployment, an MS need not scan through multiple frequency bands to search for the cell.
Chadrasekhar suggests using interference avoidance methods to reduce the outage probability. For example, each macrocell user and each femtocell may employ time-hopping in order to decrease interference. Further, the macrocell and femtocell may both use a sectored antenna reception for improving the capacity. Chandrasekhar's analytical/simulation results show that, by using interference avoidance (specifically, time-hopped code-division multiple access (TH-CDMA) and sectorized antennas), up to seven times higher femtocell BS (fBS) density can be supported in a shared spectrum network, relative to to a split spectrum network with omnidirectional femtocell antennas. However, sectored antennas may be difficult to implement at the femtocells (which are necessarily, for practical considerations, simpler devices than regular BSs). Further, a time-hoping approach increases symbol duration (and hence, decreases data rate).
Lester, discussed above, analyzed hand-off probabilities for different power configurations at a femtocell. Since the manual cell-planning used in macrocell networks is not economically practical for femtocells, femtocells typically require auto-configuration capabilities (e.g., automatic power and cell size configuration). Lester's simulations show that call drop probabilities can be significantly decreased in a residential co-channel femtocell deployment through simple pilot power adaptation mechanisms.
The article, “Performance of Macro- and Co-Channel Femtocells in a Hierarchical Cell Structure” (“Claussen”), by Holger Claussen, published in Proc. of IEEE Int. Symp. on Personal, Indoor and Mobile Radio Communications (PIMRC), pp. 1-5, September 2007, discloses a simple power control algorithm for pilots and data signals in femtocells. Simulation results show that the interference to the macrocell network can be minimized through intelligent power control techniques.
In Lester, Chandrasekhar and Clausen, relatively simple power control mechanisms are proposed for femtocells, so that the signal-to-interference ratio (SINR) is equal to 0 dB at the cell edge. However, depending on the distance between the mBS and the fBS, such power control strategies may not be effective. For example, as mentioned above, the maximum transmission power of a co-channel femtocell may not be sufficient to provide satsifactory coverage when the fBS is close to the mBS.
The article, “Home NodeB Output Power,” published by Ericsson, 3GPP TSG Working Group 4 meeting (available at http://www.3gpp.org/ftp/tsg_ran/WG4_Radio/TSGR4—43bis/Docs/), provides a power control scheme which reduces the femtocell transmit power as the distance between the macrocell BS and the femtocell BS increases. Under such an arrangement, the macrocell MSs experience better coverage as a result of reduced interference from the femtocell. However, this approach is questionable when a femtocell is either very close to or very far away from the macrocell BS.
The article, “Uplink User Capacity in a Multicell CDMA System with Hotspot Microcells,” by S. Kishore, L. J. Greenstein, H. V. Poor, and S. C. Schwartz, published in IEEE Trans. On Wireless Communications, vol. 5, no. 6, pp. 1333-1342, June 2006, overcomes the near-far effect by increasing femtocell coverage. Increased femtocell coverage is achieved by allowing an MS close to a femtocell to communicate with the macrocell BS only when the signal quality from the macrocell BS is significantly better. This approach increases interference at neighboring femtocells.
The following patent application publications disclose femtocell implementations: (a) U.S. Patent Application Publication 2007/0183427, “Access Control in Radio Access Network Having Pico Base Stations,” by T. Nylander et al., filed Oct. 3, 2006; (b) U.S. Patent Application Publication 2007/0254620, entitled “Dynamic Building of Monitored Set”, by T. L. E. Lindqvist et al., filed Apr. 28, 2006; and (c) Internation Patent Application Publication WO2006/0139460, entitled “Method and Apparatus for Remote Monitoring of Femto Radio Base Stations”, by J. Vikeberg et al., May 30, 2006. However, none of these patent applications offers an efficient frequency assignment scheme for a femtocell deployment.